1. Field of the Invention
The present invention relates to an image-blur correcting viewing-optical system which corrects blur of an image by driving an image-blur correcting lens element in a direction perpendicular to the optical axis in accordance with the magnitude of a shake imposed on the viewing-optical system.
2. Description of the Prior Art
At least two types of image-blur correcting viewing-optical systems have been known in the art:
(1) type-i: employing a variable angle prism; and
(2) type-ii: employing an image-blur correcting lens element to be driven in a direction perpendicular to the optical axis.
In the case where a variable angle prism is employed (the above type-i), since the boundary surfaces thereof exposed to the air are flat, spherical aberration is inherently occurred, thereby decentration-coma and chromatic aberration inevitably occur.
On the other hand, in the case where an image-blur correcting lens element is employed (the above type-ii), since enough space for the image-blur correcting lens element to retreat has to be provided, and a space for a drive mechanism thereof is required, the size of the entire optical system has to become larger. The shape of the image-blur correcting lens element is, in particular, designed so that the shape of the exit pupil does not change even when the image-blur correcting lens element is decentered by the maximum amount in a direction perpendicular to the optical axis. Therefore the image-blur correcting lens element has to be made larger; and still further, the thickness thereof also increases. Since the image-blur correcting lens element itself becomes heavier, a more load is imposed on the drive mechanism thereof. As a result, a drive mechanism with strong driving force is required, which causes an increase on the size of the entire apparatus, and also causes an increase on the production costs thereof.
Particularly, in the case of a small lens system, such as a binocular, in which some of lens elements in the objective optical system thereof are utilized as the image-blur correcting lens elements, it is considered to be essential to make the diameter of the image-blur correcting lens element larger than that of another lens element(s) in the objective optical system. Accordingly, considering the amount of decentering when blur of an image is corrected, and the size of the drive members for the image-blur correcting lens elements, the body containing the image-blur correcting viewing-optical system is unavoidably made larger.
The present invention is to provide a light-weight and miniaturized image-blur correcting viewing-optical system, which is attained by miniaturizing image-blur correcting lens elements which constitute a portion of an objective optical system thereof.
According to the present invention, there is provided an image-blur correcting viewing-optical system including an objective optical system, an image-erecting optical system and an eyepiece optical system. The objective optical system includes a front lens group having a positive lens element, a negative lens-element and a positive lens element, and a rear lens group constituting an image-blur correcting lens element which is driven in a direction perpendicular to the optical axis, in this order from the object. The image-blur correcting viewing-optical system satisfies the following condition:
0.5xe2x89xa6hc/hmaxxe2x89xa60.8xe2x80x83xe2x80x83(1)
wherein
hc designates the radius of an axial open aperture of the first surface of the image-blur correcting lens element; and
hmax designates the radius of an axial open aperture of the first surface of the front lens group of the objective optical system.
The image-blur correcting viewing-optical system according to the present invention satisfies the following condition:
0.5 mm less than |tan 1xc2x0/(xcfx860xe2x88x92xcfx861)| less than 5 mmxe2x80x83xe2x80x83(2)
wherein
xcfx860 designates the power of the entire objective optical system, and
xcfx861 designates the power of the front lens group.
The image-blur correcting viewing-optical system according to the present invention satisfies the following condition:
0.04 less than xcex94hc/(2xc3x97hc) less than 0.2xe2x80x83xe2x80x83(3)
wherein
xcex94hc designates the amount of correction for decentering with respect to a blur angle of 1xc2x0 on the image-blur correcting lens element.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-139884 (filed on May 10, 2001) which is expressly incorporated herein in its entirety.